1. Field of the Invention
This invention relates to a taper journal bearing for rolls for use in rolling mills.
2. Description of the Prior Art
A taper journal bearing of a roll of this kind provides a bearing surface with oil film between a bushing housed in a bearing box of a roll stand and a sleeve closely fitted on a taper journal of the roll. In this case, a key is provided between the sleeve and the taper journal of the roll to prevent a relative movement therebetween.
The bearings of this type have been extensively used, but they have often encountered a problem of variation in reduction force due to an eccentricity of a rotating axis of the roll, which has been considered to be unavoidable resulting from the inherent construction of the bearings by the use of oil films.
It will be understood that the expression "reduction force" used hereinbelow will mean a force acting upon rolls for rolling plates therebetween.
Instead of the plain bearings, cylindrical roller bearings have been used in rolling mills, which may mitigate the variations in reduction force due to the eccentricity of rotating axes of rolls. However, the durability of the roller bearings is inferior to that of the plain bearings when used at high speeds with high accuracy. This problem has not been solved. In addition, a change of the rolls with the roller bearings is very troublesome. Accordingly, the plain bearings with oil film are greatly advantageous for multi-Hi rolling mills such as hot or cold strip mills.
Under the circumstances, it has been expected to eliminate the variation in reduction force in the plain bearings with oil film and the control of the variation in reduction force acting upon roll bearing boxes has been under investigation. However, any satisfactory answer to this problem has not been obtained.
It has been found from data in work rolls of 2-Hi rolling mills and back-up rolls of 4-Hi rolling mills that the variation in the reduction force is caused every one rotation of the roll due to the eccentricity thereof and there are minus peaks in extremely steep curves of the reduction force. It is very difficult, if not impossible, to compensate these violent changes in reduction force by the present technique for controlling thickness of plates to be rolled.
In view of the fact that such violent changes in reduction force occur only in the rolling mills using the plain bearings with oil film, we have thoroughly investigated the construction of the bearing allowing the violently varying reduction force and found that it is caused by the keyway formed in the sleeve for the key for the purpose of preventing the relative movement of the taper journal and sleeve.
The keyway formed in the inside of the sleeve is somewhat deeper than the height of the key extending beyond the taper journal to form a clearance between the key and a bottom of the groove or keyway of the sleeve. The clearance is unavoidably provided in consideration of the thermal expansion of the key. Such a clearance between the key and the bottom of the groove permits an elastic deformation of the sleeve to abruptly reduce the reduction force at the moment when the keyway of the sleeve comes in registry with a plane where the sleeve is subjected to the reduction force. It has been found that an amplitude of variation in reduction force or a difference between maximum and minimum reduction forces may often reach as much as 18 tons while idling of the roll irrespective of reduction forces and rotating speeds.
This abrupt change in reduction force will adversely affect in conjunction with a high plasticity of the material to be rolled at this stage the accuracy of the thickness of plates to be rolled at relatively low rolling speed in stands on an entry side (particularly a first stand) of tandem rolling mills and in initial rolling operation of reversible mills. The unevenness in thickness of the rolled plates caused by the abrupt change in reduction force could not be completely eliminated by repeated rolling operations following thereafter.